1. Field of the Invention
The present invention relates to steel cords to be used for reinforcing articles of rubber or the like and, more particularly, to steel cords composed of nine wires or filaments.
2. Description of the Prior Art
Rubber articles such as radial tires, conveyor belts or hoses for high pressure use steel cords as their reinforcing materials. In the prior art, one type of steel cord has the "3+6 structure" consisting of a center core and an outer jacket, as shown in FIG. 4. This structure is disclosed in U.S. Pat. No. 3,858,435.
In this steel cord having the 3+6 structure, around a center core (or strand) 100 having three wires or filaments (as will be shortly referred to as the "wires") twisted, there is arranged and twisted an outer jacket (or strand) 200 having six wires. This 3+6 type steel cord is classified depending upon the twisting direction into the opposite direction type and the uni-direction type. In the former type, the center core is twisted in the lefthand (or righthand) lay, and the outer jacket is twisted in the righthand (or lefthand) lay. In the latter type, both the center core and the outer jacket are twisted in the lefthand (or righthand) lay.
In either type, however, the 3+6 type steel cord has to be manufactured at two steps, i.e., the core stranding step and the outer closing step. This necessity drops the productivity and raises the production cost. Especially, the steel cord of the opposite direction type is encountered by a phenomenon that the center core is twisted back at the outer closing step. This makes it necessary to twist the center core with a shorter pitch than that of the final cord product. Thus, the steel cord of the opposite direction type is defective in the lower production efficiency.
In the 3+6 type steel cord, moreover, the three wires composing the center core 100 are in contact with one another so that a gap E having a closed section is established in the cord center, as shown in FIG. 4. This makes it impossible for the rubber material to impregnate into the center core thereby to leave the gap E as it is, when the cord and the rubber are to be combined. As water steals into the gap E when the rubber product is used, the corrosion of the center core advances to invite a problem that the fatigue resistance of the cord is deteriorated by the wear of fretting.
The steel cord of the uni-direction type has its two layers twisted in the same direction so that the retaining force (or fastening force) of the outer jacket for the center core is weak. As a result, when the steel cord of this type is used as the tire reinforcing material, there arises a problem that the center core is displaced to come out of the cord end by the repeated compressions or tensile bendings. As counter-measures for preventing the center core from coming out, it is conceivable to make the so-called "open structure", in which the adjacent ones of the three wires or filaments composing the center core are kept away from contacting with each other. However, the center core is twisted twice, that is, once at the core stranding step in the lefthand (or righthand) lay into an open structure and then at the outer closing step in the lefthand (or righthand) lay. This relation makes the center core into the so-called "tight structure", in which the center core has a short twisting pitch so that the wires or filaments come into contact with each other. This makes it practically unexpectable to prevent the core from coming out.
Incidentally, in a known multi-layer twisted steel cord, a number of wires or filaments are twisted all at once, as in the bunched type having the 1.times.12 structure or the 1.times.27 structure. This bunched type steel cord is advantageous in its excellent production efficiency because it can be manufactured by the single twisting step.
By applying this concept, therefore, it can be conceived to make a cord having nine wires from the cord having the 3+6 structure into the cord having the 1.times.9 structure. However, the cord, which is manufactured merely by preforming and twisting the nine wires, cannot still solve the problem that the three wires corresponding to the center core are liable to come out, because the three wires at the center are in linear contact with the surrounding six wires. Since, moreover, the three wires corresponding to the center core are in close contact with each other, the rubber penetration is still insufficient and cannot be released from the problem of deteriorating the fatigue resistance.
In the two-layered steel cord having the 3+6 structure or the 1.times.9 structure, moreover, the torsion is balanced between the torque (as will be referred to as the "residual torsion") of the center core and the torque of the outer jacket. The steel cord having the 1.times.9 structure has a far higher residual torsion at its center core than that of the 3+6 structure. In case, therefore, the steel cord and the rubber are combined into a sheet article, the residual torsion of the center core disappears at the cut face (or cord terminal) so that the wires of the outer jacket have a stronger torque. As a result, the combined sheet article exhibits flatness in the regions apart from the cut face but a rise at one end in the vicinity of the cut face. If, on the contrary, the torsion is set to flatten the region near the cut face, the regions apart from the cut face get warped, as indicated at Z (having a height of 6 to 10 mm) as shown in FIG. 5. This warp of the sheet article will cause disadvantages in the sizing accuracy at subsequent cutting (bias-cutting) and jointing steps.